<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>DOM</title>
</head>
<body>
    <!-- <button onclick="changeThis()">红色</button> -->
<ul class="li">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li class="box">6</li>
</ul>
<script>
    let ele = document.getElementsByClassName('li')[0];
    let box = document.getElementsByClassName('box')[0];
    ele.style.color="red";
    box.style.color="black";
</script>
</body>
</html>